In the technology of industrial measurements and automation, inline measuring devices having a vibration-type measuring pickup are used for high-accuracy registering of measured process variables, especially flow-dynamic and/or rheological, measured variables, of media flowing in conduits, especially pipelines. Such measuring devices typically include at least one measuring tube communicating with the medium-conveying pipeline and vibrating during operation. Construction, functioning and examples of use of such measurement pickups of vibration-type are described in detail, for example, in U.S. Pat. No.-A 4,127,028, U.S. Pat. No.-A 4,524,610, U.S. Pat. No.-A 4,768,384, U.S. Pat. No.-A 4,793,191, U.S. Pat. No.-A 4,823,614, U.S. Pat. No.-A 5,253,533, U.S. Pat. No.-A 5,610,342, U.S. Pat. No.-A 6,006,609, U.S. Pat. No.-A 6,047,457, U.S. Pat. No.-B 6,168,069, U.S. Pat. No.-B 6,314,820, U.S. Pat. No.-B 6,352,196, U.S. Pat. No.-B 6,397,685, U.S. Pat. No.-B 6,450,042, U.S. Pat. No.-B 6,487,917, U.S. Pat. No.-B 6,516,674, U.S. Pat. No.-B 6,519,828, U.S. Pat. No.-B 6,523,421, U.S. Pat. No.-B 6,598,281, U.S. Pat. No.-B 6,666,098, U.S. Pat. No.-B 6,698,644, U.S. Pat. No.-B 6,711,958, U.S. Pat. No.-A 6,769,163, WO-A 03/048693, or the assignee's not pre-published German Application DE 10354373.2.
Vibration-type measurement pickups serve, as is known, to produce, in conjunction with a measuring device electronics connected thereto, reaction forces in the medium at the moment conveyed in the at least one measuring tube. These reaction forces correspond with the process variables to be measured and include e.g. Coriolis forces corresponding with a mass flow rate, inertial forces corresponding with a density, or frictional forces corresponding with a viscosity, etc. Measurement signals are then derived from these forces, appropriately corresponding with the measured process variables, for example the particular mass flow rate, viscosity and/or density of the medium. The at least one measuring tube of the measurement pickup is usually medium-tight, especially pressure-tight, for this purpose and is most often inserted permanently into the course of the pipeline conveying the medium, for instance by means of flange connections. For the oscillatable holding of the at least one measuring tube, a tubular or frame-shaped support element is provided. The support element, for example of steel, is usually made to be very resistant to bending, as compared to the measuring tube, and is mechanically coupled to the particular measuring tube, for example directly affixed thereto, at the inlet and outlet ends. The support element can, as is usual for such measurement pickups and also clearly evident from the aforementioned state of the art, be completed to form the already mentioned measurement pickup housing by means of appropriately, externally applied coverings, such as e.g. tube-covering caps or laterally applied sheets, or it can even itself be constructed as a measurement pickup housing.
For driving the at least one measuring tube, measurement pickups of the described kind additionally include an exciter arrangement electrically connected with the particular measuring device electronics. The exciter arrangement includes an oscillation exciter, especially an electrodynamic or electromagnetic oscillation exciter, acting mechanically on the measuring tube. In operation, the exciter arrangement is driven by the measuring device electronics by means of appropriate exciter signals in suitable manner such that the measuring tube at least temporarily executes vibrations, especially bending oscillations and/or torsional oscillations. Additionally, a sensor arrangement is provided for producing oscillation measurement signals and having, at least in the case of application of the measuring pickup as a Coriolis mass flow measurement pickup, at least two mutually separated sensor elements reacting to inlet and/or outlet vibrations of the measuring tube.
Besides the possibility of simultaneously measuring a plurality of such process variables, especially mass flow rate, density and/or viscosity, by means of one and the same measuring device, another essential advantage of inline measuring devices having measurement pickups of vibration-type, is that they exhibit, within specified operational limits, a very high measurement accuracy coupled with relatively low sensitivity to disturbances. Moreover, such a measuring device can be used for practically every flowable medium and applied in a multitude of the most varied application areas of the technology of measurements and automation.
In the case of inline measuring devices of the described kind applied as Coriolis mass flow meters, the particular measuring device electronics determines, among other things, a phase difference between the two oscillation measurement signals delivered by the two sensor elements and the measurement electronics issues at its output a measured value signal derived therefrom, which represents a measured value corresponding to the time behavior of the mass flow rate. If, as usual in the case of such inline measuring devices, also the density of the medium is to be measured, then the measuring device electronics additionally determines for such purpose an instantaneous oscillation frequency of the measuring tube on the basis of the oscillation measurement signals. Moreover, also, for example, the viscosity of the medium can be measured on the basis of the power required for maintaining the oscillations of the measuring tube, especially a corresponding exciter current for the exciter arrangement.
For operating the measurement pickup, especially for the further processing or evaluation of the at least one measurement signal, the measurement pickup is, as already indicated, connected with a corresponding measuring device electronics. In the technology of industrial measurements and automation, this measuring device electronics is often connected for this purpose via an associated data transmission system, e.g. via a digital data bus, with other measuring devices and/or with a remote, central computer, to which it sends the measured-value signals. Serving as data transmission systems in this case are often bus systems, especially serial bus systems, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, and the corresponding transmission protocols. By means of the central computer, the transmitted measured-value signals can be processed further and visualized as corresponding measurement results e.g. on monitors and/or converted into control signals for appropriate adjustment means, such as e.g. magnetic valves, electromotors of pumps, etc. For accommodating the measuring device electronics, such inline measuring devices further include an electronics housing, which, as e.g. proposed in WO-A 00/36379, can be located remotely from the measurement pickup and connected therewith simply over a flexible line, or which, as shown e.g. also in EP-A 1 296 128 or WO-A 02/099363, is arranged directly on the measurement pickup, especially on top of a measurement pickup housing accommodating the measurement pickup.
In the case of measurement pickups of the described kind, essentially two kinds of tube shapes have become established on the market, namely, on the one hand, essentially straight measuring tubes, and, on the other hand, measuring tubes essentially curving in a tube plane, among which those having essentially S-, U- or V-shape are most frequently used. Especially in the case of Coriolis mass flow measurement pickups serving for the measurement of mass flow rates, in the case of both kinds of tube forms, for reasons of symmetry, mostly two measuring tubes are used, which, at rest, extend essentially parallel to one another and most often are flowed through by medium also in parallel. In this connection, reference can be made, by way of example, to U.S. Pat. No.-A 4,127,028, U.S. Pat. No.-A 4,768,384, U.S. Pat. No.-A 4,793,191, U.S. Pat. No.-A 5,610,342, U.S. Pat. No.-A 5,796,011 or U.S. Pat. No.-B 6,450,042.
Besides measurement pickups with such double measuring tube arrangements, however, also measurement pickups having a single, straight or curved, measuring tube have been available for a long time on the market. Such measurement pickups of vibration-type with a single measuring tube are described e.g. in U.S. Pat. No.-A 4,524,610, U.S. Pat. No.-A 4,823,614, U.S. Pat. No.-A 5,253,533, U.S. Pat. No.-A 6,006,609, U.S. Pat. No.-B 6,314,820, U.S. Pat. No.-B 6,397,685, U.S. Pat. No.-B 6,487,917, U.S. Pat. No.-B 6,516,674, U.S. Pat. No.-B 6,666,098, U.S. Pat. No.-B 6,698,644, U.S. Pat. No.-B 6,711,958, WO-A 03/048693, or the assignee's mentioned application DE 10354373.2. Each of the measurement pickups shown therein includes, among other things, a measuring tube having an inlet end and an outlet end and vibrating, at least at times, especially a measuring tube of steel, titanium, tantalum or zirconium or corresponding alloys, for the conveying of the medium to be measured, wherein the measuring tube communicates with a connected pipeline via a first tube segment opening into the inlet end and via a second tube segment opening into the outlet end for enabling the flow-through of the medium and wherein the measuring tube executes, during operation, mechanical oscillations about an oscillation axis imaginarily connecting the two tube segments, as well as including a mostly very bending-stiff, tubular or frame-shaped, support element, for example of steel, for the oscillatable holding of the measuring tube, which is affixed to the first tube segment by means of a first transition piece and to the second tube segment by means of a second transition piece.
For the above-described case, that the measurement pickup being utilized is one involving a single measuring tube, counter oscillator means is/are additionally provided in the measurement pickup, suspended oscillatably in the measurement pickup housing and affixed to the measuring tube, in order, apart from the holding of the oscillation exciter and the sensor elements, to decouple the vibrating measuring tube from the connected pipeline as regards oscillation. The counter oscillator, which is usually made of a cost-favorable steel, can, in such case, be embodied e.g. as a tubular compensation cylinder or box-shaped support frame arranged coaxially with the measuring tube. To the referenced assembly of features of the separate, above-described, measurement pickups is still to be added that a straight measuring tube, or straight measuring tubes, is/are mostly made of pure titanium, a titanium alloy with high titanium content, pure zirconium, or a zirconium alloy with high zirconium content, since, compared with measuring tubes of stainless steel, which is, per se, likewise possible in the case of straight measuring tubes, usually shorter constructed lengths result, and that a curved measuring tube, or measuring tubes, is/are preferably made of stainless steel, although titanium or zirconium, or their alloys are also possible as material for the measuring tubes. Moreover, however, also the use of, for example, tantalum or corresponding tantalum alloys is usual as measuring tube material.
As can be derived from the above explanations without difficulty, practically each of the measurement pickups evidenced in the above-referenced state of the art has at least one composite system, especially a bimetallic composite system, which includes a first component—for example, the first or the second end piece—and a second component—for example, the measuring tube—extending at least partly through the first component along an imaginary longitudinal axis of the composite system, wherein usually the second component contacts an inner surface of the first component flushly with an outer, cylindrical surface, the inner surface being formed by the inner wall of a bore extending within the first component. Equally, however, there are also measurement pickups using a double measuring tube arrangement, such as described especially also in U.S. Pat. No.-A 5,610,342, constructed of a plurality of such, especially bimetallic, composite systems. Besides the composite system formed by measuring system and end piece, other examples of such, especially bimetallic, composite systems are especially also the connection of measuring tube and flange, or the connection of flange and measurement pickup housing; compare, in such connection, also U.S. Pat. No.-B 6,168,069, U.S. Pat. No.-B 6,352,196, U.S. Pat. No.-B 6,698,644. By way of example and as also described in U.S. Pat. No.-A 6,047,457, a circular, washer-shaped, metal body can be affixed on the measuring tube halfway between the end pieces, to serve as part of the exciter arrangement or to interact with such.
Very high requirements are placed on vibration-type measurement pickups used in industrial measuring and automation technology as regards accuracy of measurement, which usually lies in the range of about 0.1% of the measured value and/or 0.01% of maximum reading. For this, especially a very high stability of the zero point is required, as well as also a very high robustness of the delivered measurement signals, especially also in the case where environmental, clamping and/or operating conditions are significantly changing. As already extensively discussed in the mentioned U.S. Pat. No.-A 5,610,342, U.S. Pat. No. 6,047,457, U.S. Pat. No.-A 6,168,069, U.S. Pat. No.-B 6,519,828, U.S. Pat. No.-B 6,598,281, U.S. Pat. No.-A 6,698,644, U.S. Pat. No.-B 6,769,163, WO-A 03/048693, or the mentioned application DE10354373.2 of the present assignee, in such case, considerable importance is given also to the mechanical strength, especially fatigue strength, with which the separate components of the aforementioned composite systems formed in the measurement pickup are affixed to one another. Already the slightest departure of the strength of the aforementioned composite systems from the situation existing during calibration can result in significant, no longer manageable, fluctuations of the zero point and, consequently, in practically unusable measurement signals. Usually, such zero-point errors attributable to loss-of-strength phenomena in the composite systems can only be removed by complicated repair measures performed remote from the pipeline or only by installation of a new, inline, measuring device. Having a special influence on the stability of the zero-point and/or the availability of the measurement pickup is, in such case, as, in fact, also already discussed extensively in U.S. Pat. No.-A 5,610,342, U.S. Pat. No.-A 6,047,457, U.S. Pat. No.-B 6,168,069, U.S. Pat. No.-A 6,598,281, U.S. Pat. No.-B 6,634,241 or also WO-A 03/048693, the manner in which the measuring tube is secured within the outer support element and relative to the possibly present counter oscillator.
Traditionally, the components of such composite systems are at least partly bonded together by solder, braze and/or weld connections. Thus, it is, for example, already described in U.S. Pat. No.-A 4,823,614, that each end of the one measuring tube is inserted into a respective bore of an inlet or outlet endpiece and affixed therein by welding, soldering or brazing front and back; compare the material beads visible in some of the figures. The endpieces are then, in turn, affixed in the outer support element. Further examples of such composite systems with bonded connections are shown in, among others, also in U.S. Pat. No. 6,168,069, U.S. Pat. No.-B 6,352,196, U.S. Pat. No.-B 6,519,828, U.S. Pat. No.-B 6,523,421, U.S. Pat. No.-B 6,598,281, U.S. Pat. No.-B 6,698,644 or U.S. Pat. No.-B 6,769,163.
As described in U.S. Pat. No.-A 5,610,342, the heat needed for the mentioned welding, soldering or brazing leaves behind, following cooling, residual stresses at the locations of the joints between the measuring tubes and the end pieces which can lead to stress corrosion cracking, which can, to a greater or lesser degree, weaken the joints and/or the material of the measuring tube. As a further problem with such bonded, weld, solder or braze connections, also material-wearing, oscillatory rubbing in the area of the joints is mentioned in U.S. Pat. No.-B 6,519,828 or U.S. Pat. No.-B 6,598,281. Moreover, as can perceived on the basis of U.S. Pat. No.-A 6,047,457, U.S. Pat. No.-B 6,168,069, U.S. Pat. No.-B 6,352,196, U.S. Pat. No.-B 6,598,281, U.S. Pat. No.-B 6,634,241, U.S. Pat. No.-B 6,523,421 or U.S. Pat. No.-B 6,698,644, especially in the case of bimetal composite systems, thus those systems where the first component and the second component are different metals, for example steel and titanium, problems can arise as regards the long-term strength, for instance fatigue strength, of the solder connections, which problems can be attributed, among other things, to insufficient wetting and/or radially directed, alternating, mechanical loading of the joints. As a result of this, often a lessening of the nominal pull-out strength of the composite system, measured in the direction of its longitudinal axis, is to be noted.
For improving the long-term strength of such composite systems, for example formed of a measuring tube of a Coriolis mass flow rate measurement pickup and a metal body pushed onto the measuring tube, and then affixed thereon, the already-mentioned U.S. Pat. No.-A 5,610,342, as well as also WO-A 03/048693, disclose a securement method for measuring tubes in end pieces, in which each end of the measuring tube is inserted into a corresponding bore of an inlet or outlet, end piece and pressed, by means of a rolling tool placed in the end, against the inner wall of the bore, especially without addition of heat, whereby a high-strength, friction connection is formed between the first and second components. A rolling tool suited for this method is described, for instance, also in U.S. Pat. No.-A 4,090,382, in the context of a method for manufacturing boilers or heat exchangers.
A further possibility for manufacturing such composite systems formed by means of high-strength, friction connections includes, as e.g. proposed also in U.S. Pat. No.-A 6,047,457, that the first component, after having been pushed, or inserted, onto the second component, is compressed by means of an externally applied pressing tool and, in the process, deformed mixed plastically-elastically below a recrystallization temperature of the component-material, especially at room temperature. The deformation forces used therefor are, in such case, always developed such that the second component essentially does not experience any cross sectional tapering and/or narrowing, so that an initial inner diameter of the second component remains essentially unchanged throughout, following the production of the composite system. An apparatus appropriately suitable for the pressing is disclosed, for example, in U.S. Pat. No.-A 3,745,633. Alternatively to the plastic-elastic pressing, such a composite system formed by means of frictional locking can, for example, also be manufactured by processing wherein the first component, as also shown in U.S. Pat. No.-B 6,598,281 or U.S. Pat. No.-B 6,519,828, is thermally shrunk onto the second component or clamped to the second component with the interposing of elastically deformable clamping elements.
Taking the subject further, U.S. Pat. No.-B 6,598,281 or U.S. Pat. No.-B 6,519,828 indicate that, with press connections holding purely by friction, a possible deterioration of the composite systems can not always be avoided with certainty, due to oscillatory rubbing. Beyond this, such oscillatory rubbing can bring about the corrosion of the materials of the composite system in the area of the mutually contacting surfaces. Furthermore, as can be perceived from WO-A 03/048693, the usually differing expansion characteristics of the components of the above-described composite systems, for example thus the above-mentioned end pieces and the tubular segments of the measuring tube clamped therein, can lead to the clamping forces exerted by the first component on the second component falling below a critical value in the face of temperature fluctuations, especially in the case of possible temperature shocks, such as can e.g. arise during regularly performed cleaning measures with extremely hot washing liquids. This can, in turn, mean that the first component and the second component lose, at locations, the mechanical contact brought about by the rolling, pressing or shrinking, due to thermally related expansions, so that the composite system can be weakened to an unallowable degree. As a result, the pull-out strength of the composite system can sink, so that the desired high zero-point stability of the measurement pickup can also not, without more, be assured with such press-joined assemblies.
For overcoming the deficiency in composite systems of the described kind caused by oscillatory rubbing between the components, it is proposed in U.S. Pat. No.-B 6,598,281 or U.S. Pat. No.-B 6,519,828 to additionally weld the associated components together, following production of the press-joined assembly, especially with the use of a filler material serving as an interposed layer. However, this can possibly bring up again the above-mentioned problems associated with welded connections. In contrast, a composite system is proposed in WO-A 03/048693, which obtains an increased twist resistance by forming a groove in the inner wall of the first component extending in the direction of the longitudinal axis of the composite system. With the formation of an interlocking connection effective in a circumferential direction, this can effectively prevent a twisting of the first component relative to the second component. However, even this composite system can experience a lessening of its nominal pull-out resistance, be it due to oscillatory rubbing and/or thermally-related expansion, especially in the case of use in a measurement pickup with a measuring tube executing, at least at times, bending oscillations.